In 1998 the United States Golf Association (USGA) limited the coefficient of restitution (COR) in drivers to 0.830, more specifically to 0.822 with a test tolerance of 0.008 effectively taking the limit up to 0.830. With the introduction of these limits, The USGA needed a test procedure to measure a driver's COR. Originally, a ball was fired by air cannon into a specimen and pre and post impact velocities were compared to find COR, as set forth in “Procedure for Measuring the Velocity Ratio of a Clubhead for Conformance to Appendix II, (5a.),” Revision 3 Jan. 1, 2002. This process took a significant amount of time to perform when considering the set up and the controls associated with the test.
Those skilled in the art know that today the characteristic time, often referred to as the CT, value of a golf club head is more widely used in determining conformance with the USGA equipment rules. The rules currently state that the characteristic time, or CT, of a club head shall not be greater than 239 microseconds, with a maximum test tolerance of 18 microseconds. Thus, it is common for golf clubs to be designed with the goal of a 239 microsecond CT, knowing that due to manufacturing variability that some of the heads will have a CT value higher than 239 microseconds, and some will be lower. However, it is critical that the CT value does not exceed 257 microseconds or the club will not conform to the USGA rules. The USGA publication “Procedure for Measuring the Flexibility of a Golf Clubhead,” Revision 2.0, Mar. 25, 2005, is the current standard that sets forth the procedure for measuring the characteristic time. However, this CT test is only applicable to drivers and the USGA still employs the air cannon COR test when testing fairway woods, hybrids and irons.
Another common golf club performance variable is the smash factor, or SF. The smash factor is defined as the ratio of ball speed to club head speed and may be used as a measure of the quality of ball striking. First, one must look to the ball speed formula, specifically:Vball=Vch*[(1+COR)/(1+(Mball)/(Mch))]*cos(loft)*(1−0.14*miss)
where:                Vball is the velocity of the ball immediately after impact with the club head.        Vch is the velocity at of the club head at impact with the golf ball.        Mball is the mass of the golf ball, most commonly approximately 46 grams.        Mch is the mass of the club head in grams.        “miss” is the distance, in inches, that impact misses the face center.        COR is the coefficient of restitution measured at the face center.        
Since the smash factor (SF) is the ratio of the ball speed to the club head speed, the equation for smash factor (SF) is:SF=(Vball/Vch)=[(1+COR)/(1+(Mball)/(Mch))]*cos(loft)*(1−0.14*miss)
Thus, the potential smash factor (SF) at any point on the face can be determined and is very club head specific since it takes into account the particular golf club head's COR, mass, and loft.
Currently, most golf club heads that are sold to the general golfing population are not individually tested to determine each golf club head's CT, or COR, or potential SF value(s). Generally, a random sample will be taken off the manufacturing line and tested to determine conformance of a much larger lot of heads. Thus, it is not uncommon for golf club heads having nonconforming CT, or COR, values to make it to the market. In fact, in the past decade at least two major golf equipment companies have issued recalls on their products because heads with higher than allowable CT value have made it into the hands of golfers. Such product recalls are incredibly expensive and result in tremendous waste, not to mention the impact upon consumer confidence.
Numerous production variables influence the final COR and CT values of a golf club head, as well as the potential SF values. Today, most driver heads are composed of several individual components joined together to create a golf club head. Often times the golf club's sole, crown, and face are distinct components, often made of dissimilar materials and/or thicknesses, which are joined together to create the final product. Generally the metallic components are welded or brazed together, and the non-metallic components are joined to metallic components by adhesives. One skilled in the art will appreciate that manufacturing variables such as the quality of a weld, the amount of brazing material, and adhesive quality and curing, just to name a few, can influence the final performance of a golf club head, including the COR, CT, and potential SF values. Further, variability in the composition of the alloys used to form the various components may influence the performance variables, as can unintended variations from the design thickness of the components introduced during casting or forging. Because of these factors, one golf club head design that is produced at two different manufacturing facilities may have significantly different COR and CT values. Thus, the real COR and/or SF and/or CT value of a golf club head cannot be accurately determined until the head is fully assembled in its final form.
A consumer purchasing a golf club often has no idea if the particular golf club head they are purchasing has a CT value or COR value within the USGA limit, above the limit, or well below the limit. Utilizing a club head with a value above the limit during tournament play could result in the disqualification of a golfer; whereas using a club head with a value below the limit means that the golfer is possibly handicapping themselves with respect to the rest of the field. Additionally, the purchaser generally has no idea how the CT, COR, and/or potential SF values vary at particular locations across the face of the golf club head. Therefore, the need exists for a method of identifying and marking the CT, COR, or potential SF value on the face of a golf club head, either permanently or via a removable face template.